IQ-imbalance, or IQ-mismatch, is a well-known imperfection in receivers used in wireless as well as wired communications. There are several reasons why IQ-imbalance may occur. A common reason for the occurrence of IQ-imbalance in a receiver is when a signal is directly down-converted from radio frequency (RF) to baseband frequency. An arrangement for such down-conversion is referred to as a zero-intermediate frequency receiver or a homodyne receiver. With this kind of arrangement, the RF signal is typically down-converted using a quadrature mixer, i.e., a mixer with two parallel mixing stages, where the RF signal is in the analog domain multiplied by two sinusoids with a 90-degree phase difference (for instance a cosine and a sine) to result in two output signals, respectively.
Due to limitations in the analog implementation of the quadrature mixer, the output is not ideal. Specifically, there is typically a phase error, φ, such that the phase difference between the two output signals will not be exactly 90 degrees but rather (90+φ) degrees. The phase error φ might also depend on local oscillator (LO) imperfections. Whatever reason, this is referred to as phase mismatch. Furthermore, the gains of the two mixing stages will not be perfectly matched. This is commonly referred to as gain mismatch.
The resulting IQ-mismatch is often compensated in a feed-forward fashion, that is, any imperfection in the analog components of the receiver is compensated in the digital domain. This approach has shown to be an effective means to allow for relaxed requirements for analog components. However, it comes with a cost of additional digital processing.
One promising receiver architecture uses an intermediate frequency (IF), rather than a zero-IF, approach. An example of such a receiver architecture is the dual-carrier complex IF (CIF) based receiver as depicted in FIG. 1. In this example, the receiver is designed to process two carriers simultaneously.
As shown in FIG. 1, the received RF signal is first fed to a low-noise amplifier (LNA). The amplification is then followed by a quadrature RF down-conversion mixer where the local oscillator frequency is typically set to approximately halfway between the two carriers such that the two carriers will be placed on the same absolute IF frequency. IF filters (IFF) may be used to filter out these carriers while suppressing interfering signals. A complex IF mixer, such as to the dual carrier complex mixer shown in the middle of FIG. 1, is used to down-convert the two carriers to baseband, after which channel select filtering (CSF) and analog-to-digital conversion (ADC) are performed. Note that the illustration of FIG. 1A is a simplified view of the receiver architecture with the purpose only to exemplify the basic operations.
As explained above, quadrature mixers suffer from gain and phase imbalance between the generated in-phase (I) and quadrature-phase (Q) signals. The performance in this respect is usually measured as image-rejection ratio (IRR). For RF quadrature mixers the IRR is typically in the range of 30-40 dB. In the exemplary architecture described above a finite image rejection in the RF path will cause signal leakage from the lower side carrier into the upper side carrier and vice versa. Gain and phase imbalances will also be introduced by the IF filters and the IF mixer. Typically, the IF mixer has a better IRR than the RF mixer since the IF mixer operates with much lower frequencies.
The IQ-imbalance caused by the RF mixer may be compensated by an adjustable IF mixer. The assumption for this to be valid is that the IQ-imbalance is frequency independent, i.e., the imbalance is the same for the entire bandwidth of the desired signal. For a mixer circuit this assumption can be justified when the bandwidth is sufficiently small compared to the carrier frequency, say, in the order of 1%. It has, however, been found that when the IQ-imbalance is additionally caused by mismatch between other components, such like filters, the resulting IQ-imbalance may no longer be fully frequency independent.
The adjustable IF mixer as mentioned above may only enable the removal of the frequency flat part (i.e., frequency-independent portion) of the IQ-imbalance. When this does not suffice the result can be that the supported data rate has to be reduced. If instead the IQ-imbalance is compensated in the digital domain, higher requirements may be imposed on the ADC, especially when a strong interferer is present at the image frequency. In this case, compensating in the digital domain, if possible at all, may lead to increased power consumption and increased cost because of the higher requirements on the ADC.